Photodetectors and light emitting diodes (LEDs) in the wavelength range from 1.55 to 1.3 xcexcm are used in telecommunication by means of glass fibers. At present, use is principally made of components based on III-V semiconductors which are relatively expensive.
Some proposals have been made relating to Si1-xGex/Si semiconductor components for use in this wavelength range and have been reported by H. Presting et al. in Semiconductor Science Technology 7, pages 1127 and sequel of 1992 and in App. Phys. Lett., Vol. 66 No. 17, Apr. 24, 1995 in the article xe2x80x9cStrained Si1-xGex multi-quantum well waveguide structure on (110) Sixe2x80x9d by K. Bernhard Hxc3x6fer, A. Zrenner, J. Brunner and G. Abstreiter.
However, substantial problems exist with such Si1-xGex/Si structures because Ge has a lattice constant which differs substantially from that of Si. The mechanical strain which thereby results in structures of this kind makes it necessary to restrict the thickness of the layers to an extent which places severe constraints on the use of the Si1-xGex/Si material system.
Proposals have also been made relating to light emitting diodes, realized by a p-n junction formed in silicon carbide.
That is to say, the junction is formed by the transition from a p-SiC substrate to an n-SiC layer, with the contacts being provided at the two layers. One proposal of this kind is to be found in DE-A-23 45 198. A further discussion of this system is also to be found in DE-A 39 43 232 which states that SiC based LEDs are disadvantageous in comparison with various LEDs based on III-V or II-VI material systems. The reason given is that SiC has the disadvantage that the light yield for a p-n LED is low because SiC is a material with an indirect band gap. The document concludes that SiC LEDs cannot therefore be used for practical applications.
Finally, for the sake of completeness, reference should be made to DE-A-35 36 544 which discusses the deposition of a layer of semiconductor material from the gas phase onto the and of a glass fiber to form a detector. This is intended as a particularly simple way of coupling out the light from the glass fiber while simultaneously converting it into an electrical signal. It is stated that amorphous (hydrogen-containing) Si and amorphous compounds of Si with Ge, carbon or tin, amorphous Si carbide or Si nitride can be used as a semiconductor. This reference is not considered relevant to the present teaching, which is concerned with single crystal material.
The present invention is based on the object of providing semiconductor components in the form of photodetectors, light emitting diodes, optical modulators and waveguides which can be grown on a silicon substrate at favorable cost, which permit adjustment of the effective band gap, which enable a pronounced localization of electrons and holes, which do not require the use of complicated relaxed buffer layers, which bring about enhanced optical absorption and emission and allow these parameters to be influenced and which, in certain structures, permit the optical absorption and emission to be changed (modulated) in energy and amplitude by the application of a voltage.
In order to satisfy this object, there is provided a semiconductor component, such as a photodetector, a light emitting diode, an optical modulator or a waveguide formed on an Si substrate, characterized in that the active region consists of a layer structure with Si1-yCy, Si1-xGex, and/or Si1-x-yGexCy alloy layers or a multi-layer structure built up of such layers.
More specifically, the present invention relates to a semiconductor component having any one of the following structures:
a) a single layer of Si1-yCy 
b) a superlattice comprising alternating layers of Si and Si1-yCy 
c) a superlattice comprising alternating layers of Si1-yCy and Si1-xGex 
d) a superlattice comprising alternating layers of Si1-yCy and Si1-x-yGexCy, with the atomic fraction y of the Si1-x-yGexCy layers being equal to or different from the atomic fraction y of the Si1-yCy layers
e) a superlattice comprising a plurality of periods of a three-layer structure comprising Si, Si1-yCy and Si1-xGex layers
f) a single layer of Si1-x-yGexCy 
g) a superlattice comprising alternating layers of Si and Si1-x-yGexCy and
h) a superlattice comprising a plurality of periods of a three-layer structure comprising Si, Si1-yCy and Si1-x-yGexCy layers, with the atomic fraction of y of the Si1-x-yGexCy layers being equal to or different from the atomic fraction y of the Si1-yCy layers.
An important recognition underlying the present invention is namely that Si-based multilayer or superlattice structures with Si1-yCy/Si1-xGex/and/or Si1-x-yGexCy alloy layers open up the possibility of tailoring the lattice constants, the band gap and the shape of the band edges for the various semiconductor components.
In particular it has been found that it is possible to grow both Si1-xGax/Si1-yCy and Si1-x-yGexCy/Si1-yCy multilayer structures which are nearly compensated in average strain, and which do not suffer from deterioration of their properties due to strain relaxation. It has been found that with multilayer structures with at least double quantum wells, surprising properties are obtained which are considerably enhanced in comparison to the properties obtained with single quantum wells. Thus, for example, an improved photoluminescent efficiency has been found for Si1-x-yGexCy/Si1-yCy double quantum wells embedded in Si when compared to single quantum wells. This enhancement is considered to be quite remarkable considering the small overlap of the charge carrier wave functions. Considerably higher photoluminescent transition rates are achieved with short period Si1-x-yGexCy/Si1-yCy superlattice structures. Further enhanced photoluminescent transitions and an efficient capture of excited carriers even at room temperature can be expected for larger Ge and C contents, which appear to be practicable.
From experiments conducted to data it appears that photoluminescence can be achieved at the wavelengths of particular interest for optical fiber transmissions, i.e. in the range from 1.55 to 1.3 xcexcm (corresponding to 0.7 to 0.85 eV) and that the efficiencies which can be achieved will competitive with those of existing LEDs based on III-V or II/Vi material systems. Moreover, since the photoluminescent devices proposed here are based on Si, they should be readily acceptable and less expensive to produce, making use of known Si processing technology.
Hitherto the carbon required for the semiconductor components proposed here has been obtained from a graphite filament. It is believed that higher carbon concentrations will be achievable and the process will be better controllable in future. Carbon may also be deposited from the gas phase using suitable carbon-containing gases, such as methane or propane in a chemical vapor deposition system (CVD).
The atomic fractions x of Ge and y of C in the Si1-x-yGexCy layers and of y in Si1-yCy layers may be chosen in accordance with the guidelines given in claims 3 to 6. The values given there enable the realization of semiconductor components with beneficial properties in the sense of satisfying the objects outlined above.
The Si1-x-yGexCy, Si1-yCy and Si1-xGex layers are all substantially undoped, i.e. if dopants are present, they are due to impurities which cannot be avoided in practice. They are not, however, usually intentionally added to modify the properties of the devices under discussion.
The useful thicknesses of the alloy layers proposed in the present application in multi-layer and superlattice structures has generally been found to lie in the range from about 0.5 nm to about 10 nm.
When realizing the semiconductor components using superlattice structures, which have particularly beneficial properties, then the superlattice structure should preferably have a minimum of 10 periods and should generally not exceed more than 100 periods because of the additional cost of manufacturing such components. Favorable properties are obtained with 2 or more periods of the superlattice structures, and generally 25 to 50 periods is considered sufficient.
All the semiconductor components to which the present invention is directed, i.e. optical detectors, light emitting diodes, optical modulators and optical waveguides, can be realized using the same basic layer system.
Preferred variants for the optical detector are set forth in claims 14 and 15 and preferred variants for a light emitting diode having essentially the same structure but with reversed bias polarity are set forth in claims 16 and 17. Preferred optical modulators are achieved using basically the same layout as for an optical detector but with intrinsic layers of Si between each period of the superlattice.
For modulation to occur, the modulating voltage is, for example, applied to the positive pole, which enables the level of absorption of photons within the structure to be varied in proportion to the applied voltage.
The same basic structure as is used for all three devices can also be used as an optical waveguide and it is fortuitous that the refractive indices of the individual layers are such that light propagation parallel to the layers is possible with low losses. The ability to realize all four structures using the same or similar layer systems means that it is eminently possible to realize any desired combinations of the four structures on one chip. Thus a photodiode, an optical modulator and an optical detector could all be placed on one chip, with optical waveguides serving to transfer the light from the photodiode to the optical modulator and from the optical modulator to the optical detector. In addition, existing Si technology can readily be used to realize an infinite variety of different circuits on the same chip so that the optical devices of the present invention can be combined with all kinds of signal processing and generating circuits.
Furthermore, the structures proposed herein can readily be adapted to cooperate directly with optical fibers. Thus, as set forth, for example, in claim 23, a preferably V-shaped blind channel can be formed in the semiconductor component, with the blind channel having an end face transverse to an elongate direction of the channel and formed by a side face of an active region of the component itself, with the channel being adapted to receive one or more optical fiber ends for coupling an optical fiber or fibers to the relevant component or device.
Preferred embodiments of the invention are set forth in the claims and in the following description.
The invention will now be described in more detail by way of example only and with reference to specific embodiments as shown in the drawings.